Confusion on color2label.py script

I am confused on how did you identify the values of image[:, :, :] that corresponds to each land cover class?

    cat_image[image == 3] = 0  # (Cyan: 011) Urban land
    cat_image[image == 6] = 1  # (Yellow: 110) Agriculture land
    cat_image[image == 5] = 2  # (Purple: 101) Rangeland
    cat_image[image == 2] = 3  # (Green: 010) Forest land
    cat_image[image == 1] = 4  # (Blue: 001) Water
    cat_image[image == 7] = 5  # (White: 111) Barren land
    cat_image[image == 0] = 6  # (Black: 000) Unknown

And also, why is there no image == 4 ?

GeneralLi95 / deepglobe_land_cover_classification_with_deeplabv3plus

Confusion on color2label.py script #13